1. Field of the Invention
The present invention relates to a clamping mechanism of a molding machine.
2. Description of the Related Art
A conventional clamping mechanism of a molding machine (e.g., an injection molding machine) generally includes a stationary platen and an end frame (also referred to as a rear platen), both mounted upright on a bed of the molding machine, a plurality (normally, four) of tie bars bridging between the stationary platen and the end frame, a movable platen having through-holes individually receiving the tie bars and arranged movably along the tie bars between the stationary platen and the end frame, and a drive unit (e.g., a toggle unit) for driving the movable platen. A movable mold is attached to the movable platen, and a stationary mold is attached to the stationary platen. The drive unit including a drive source, such as an electric motor or a hydraulic cylinder, moves the movable platen, guided by the tie bars, whereby the molds are clamped. In general, a pair of sliding bearing bushes is disposed in each through-hole formed in the movable platen, and each tie bar slidably penetrates through the bushes.
In the conventional clamping mechanism as described above, the sliding bearing structure, used in a mutually sliding portion between the movable platen and each tie bar, inevitably creates a clearance between the inner circumferential surface of the bearing bush and the outer circumferential surface of the tie bar, which results in a play in the sliding bearing. Therefore, the movable platen is liable to tilt on the tie bars during a moving operation. As a result, there is a tendency that a predetermined degree of parallelism between the movable mold attached to the movable platen and the stationary mold attached to the stationary platen is deteriorated due to the moving operation of the movable paten. For example, in a mold opening process, if the parallelism between the molds is deteriorated at the instant when a mold release is caused by the separation of the movable mold from the stationary mold, the mold release timing of a molded article becomes irregular at certain points in the molding surface of the stationary or movable mold. In a case where a high-precision molded article, such as a lens, is to be molded, an irregularity in the mold release timing may result in a strain in the molded article, due to tensile stress, etc., and thus may produce a defective molded article.
In order to solve the above problem, a solution has been known, wherein a linear guide (an LM guide (trade name)) is arranged on the bed of the molding machine for guiding the travel of the movable platen, so that the movable platen is prevented from tilting and the parallelism between the movable platen and the stationary platen is maintained. In the solution using the linear guide, the movable platen is supported at the bottom end thereof by the linear guide, so that the tilting in the lower portion of the movable platen is suppressed, but it is difficult to suppress the tilting in the upper portion of the movable platen. Further, the linear guide can prevent the movable platen from turning about an axis in a moving direction of the movable platen (or an axial direction of the tie bar), but it is difficult for the linear guide to prevent the movable platen from turning about another axis perpendicular to the above axis and parallel with the mold attaching surface of the platen.
On the other hand, in a mold clamping process, the stationary platen and/or the movable platen may be deformed or strained due to a mutual pressing force applied to the stationary and movable molds. If the stationary platen and/or the movable platen is subjected to a strain, the mold-attaching surface of the platen may be deformed, which may also result in a deterioration of the parallelism between the stationary and movable molds and, thus, in a deterioration of a molding accuracy.
As a solution to prevent such a strain or deformation in the platen, for example, Japanese Utility Model Publication No. 2587035 (JP2587035Y2) discloses a clamping mechanism of an injection molding machine, wherein at least one of a stationary platen and a movable platen is provided, at an intermediate location as seen in a platen-thickness direction in the regions of tie-bar insertion holes formed at four corners of the platen, with a notch extending from the outer circumference of the platen to the tie-bar insertion holes. According to this arrangement, a pressing load applied to the molds is absorbed by a deformation in the notched region of the platen, and thus a deformation in the mold-attaching surface of the platen is prevented.
Also, Japanese Unexamined Patent Publication (Kokai) No. 8-258103 (JP8-258103A) discloses a clamping mechanism of an injection molding machine, wherein a support structure of the stationary platen is formed into a tapered shape, such as a rectangular pyramid or a truncated cone. According to this arrangement, a pressing load generated during a mold clamping is prevented from acting as a bending moment on the support structure, due to the tapered shape of the latter, so that a deflection or deformation in the support structure and the stationary platen is prevented.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-170322 (JP11-170322A) discloses a clamping mechanism of an injection molding machine, wherein a strain-preventing portion (such as a lateral edge channel) is provided, on the movable platen, between the pivot of the arm of a toggle mechanism and the mold-attaching surface. According to this arrangement, a pressing load generated during a mold clamping is absorbed by a deformation in the strain-preventing portion of the movable platen, and thus a deformation in the mold-attaching surface is prevented.
In the above conventional arts, a structure absorbing a pressing load generated during a mold clamping process is provided in the platen or the support structure thereof to prevent the platen from being strained during, especially, the mold clamping. In contrast, Japanese Patent Publication No. 3330578 (JP3330578B2) discloses a clamping mechanism of a molding machine, such as an injection molding machine, wherein a ball spline structure is adopted, in place of a sliding-bearing structure, to a guiding portion for the movable platen on the tie bar. According to this arrangement, a guide having the ball spline structure makes it possible for the movable platen to be always accurately translated (or perform a parallel displacement) relative to the stationary platen, so that a parallelism between the stationary mold and the movable mold during a mold opening process can be stably maintained. In this arrangement, it is required that the life of the ball spline structure is long even under the influence of a platen stress due to the pressing load during the mold clamping.